Molecular properties of AdoMet synthetase and enzymes of transmethylation are being characterized. Guanidoacetate methyltransferase has been purified. Guanidoacetate methyltransferase has been purified about 140-fold from pig liver. Polyacrylamide gel electrophoresis of the purified enzyme showed four protein band, each of which is associated with guanidoacetate methyltransferase activity. During gel electrophoresis at pH 3 in 8 M urea, guanidoacetate methyltransferase migrated as a single component. The molecular weight of the purified guanidoacetate methyltransferase was estimated to be 31,000 by sodium dodecyl sulfate-gel electrophoresis, which also showed only one protein component with guanidoacetate methyltransferase activity. Guanidoacetae methyltransferase is inhibited by adenosylhomocysteine, 3-deazaadenosylhomocysteine, and sinefungin with Ki values of 16 microns, and 18 microns, respectively.